What
is intercooling, and how is this system maintained? K.L.,
via e-mail

Because
the compressed air leaving a turbocharged engine can reach temperatures
of 300°F, this creates two problems. First, hot intake air raises
cylinder temperatures, which can damage internal components, especially
valves and pistons. Second, hot air is less dense than cool air, so while
a turbocharger may be pumping away, the actual volume of air that goes
into the cylinder is less than it could be.

Intercooling--sometimes
known as aftercooling--cools the air between the time it leaves the
turbocharger and the time it enters the valve passages. After air exits
the turbocharger, it passes over a series of tubes and fins containing
engine coolant or raw water. In most applications a coolant is diverted
to the intercooler right after it leaves the raw-water heat exchanger.
This is when it is at its lowest temperature, around 180°F, thus providing
a substantial temperature reduction in the air. If sea water is used,
the temperature is usually between 70 and 80°F, and so the temperature
drop is even greater, allowing more fuel to be added and more horsepower
to be produced.

Intercooler
maintenance usually involves periodic inspection of the raw-water intakes,
hoses, clamps, thermostat, raw-water pump and impeller as well as making
sure the coolant (if used) is the right mixture. Because raw-water intercooling
is more prone to corrosion and clogging, the raw-water intake strainer
should be periodically checked for debris, especially eel grass.

My
boat has an older gasoline-powered stern drive that stalls out after high-speed
cruising. If I let it cool down, it will restart, but as soon as I begin
running at high speed, it stalls again. What can be wrong? F.S.,
via e-mail

This sounds
like vapor lock, which can be caused when the fuel temperature soars due
to an overheated engine or running in hot weather. When the gasoline gets
too hot in the carburetor bowl, it ceases to flow into the venturis, and
the engine basically starves. In extreme cases fuel reaching the fuel
pump overheats, and the pump can't handle the resulting vapor. Again,
the engine is starved for fuel.

If your
engine is overheating, check your cooling system, including the level
and mixture of engine coolant (for freshwater-cooled models), thermostat,
fan belt, and fuel pump. If your engine is not overheating, your only
solutions are insulting the fuel line and changing to a fuel with a special
summer "anti-vapor lock" blend.